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Question DetailsAsked on 9/7/2015

Is it ok for insulated siding to go over Fiber Cement siding (w/asbestos) without house wrap or vapor barrier in NY

House is located in Long Island, NY. I know siding over Fiber Cement siding w/asbestos is common here in LI. But our contractor is saying that the house will need to breath since we are putting insulated siding (Alside Prodigy) over the existing siding so no house wrap or vapor barrier is needed. Any advise is greatly appreciated. Thank you in advance!

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1 Answer


I am going to assume you are putting lap plank siding over sheet cement board siding.

Basically speaking (with a few special exceptions) NO siding should go up without a housewrap - in your case because going over another siding, should have an airgap separating it from the underlying fiber cement board using firring strips AND a breathable housewrap, then the new siding goes over that.

IF the existing siding has a housewrap or tarpaper under IT, then those should come off before putting on the new siding or you risk trapping moisture between the two water barriers and promoting mold.

Failure to provide a water barrier behind siding is asking for trouble.

IF what you have is existing lap siding WITH a water barrier under it and you are putting new lap siding directly over that, other than that I think that kind of no-gap overlay is a bad idea in the first place and commonly results in unsightly installations with waves and sways and sags, then another water barrier would be a bad idea. But expect the possibility (or probability maybe) that you will develop mold between the two layers, and possibly rust or copper staining from the underlying layer's fasteners coroding from the trapped moisture - especially in your seashore location. You are assuming the prior layer AND its water barrier will perform well - which considering you are overlaying them with new siding sounds a bit like an oxymoron - or a hope and a prayer situation because if that were true then you would not be re-siding.

Answered 5 years ago by LCD

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